This invention relates to interferometers and particularly to a method and means for measuring the relative angle between two subject mirrors. The two subject mirrors may be fixed, or if they are not, this invention will measure both the relative angle and changes in the relative angle between the two subject mirrors.
Conventional interferometric techniques have long been used to measure relative characteristics of optical surfaces, but in general, the techniques have always compared the surface being measured with a reference surface. It has always been difficult to measure or compare two surfaces on the same object. The one exception might be the measurement of two surfaces of a transparent object where one of the interfering beams is transmitted through the object to one of the surfaces (Fabry-Perot). A recent invention U.S. Pat. No. 4,056,323) permitted for the first time the measurement of two parallel opaque surfaces such as laser rods or guage blocks. That device is currently being marketed by Space Optics Research Labs in Chelmsford MA under the name Ludman Interferometer (L-I). That device is well suited to measuring parallelism but not for the measurement of any other angles. The measurement of other angles is very important for applications such as optical scanners or readers and also for a wide variery of high tolerance machine parts. The invention described here uses the L-I as one of its components and is capable of measuring any angle between two subject mirrors.